Separators for use in electrochemical devices, in particular in secondary batteries, mainly serve to physically and electrically separating the anode from the cathode of the electrochemical cell, while permitting electrolyte ions to flow there through.
Separators must be chemically and electrochemically stable towards the electrolyte and the electrode materials and must be mechanically strong to withstand high tensions generated during battery assembly operations.
Further, their structure and properties considerably affect battery performances, including energy density, power density, cycle life as well as safety.
For high energy and power densities, the separator is required to be very thin and highly porous while still remaining mechanically strong.
For battery safety, the separator should be able to shut the battery down when overheating occurs so that thermal runaway, causing dimensional shrinking or melting of the separator, which results in physical contact of the electrodes, and the resulting internal short circuit can be avoided.
Also, a low thickness of the separator is required for high energy and power densities. However, this adversely affects the mechanical strength of the separator and the safety of the battery thereby provided.
Inorganic composite membranes have been widely used as separators for electrochemical devices including secondary batteries, in particular Lithium-ion batteries.
A variety of inorganic filler materials have been long used to fabricate inorganic composite membranes wherein inorganic particles are distributed throughout a polymeric binder matrix.
Although inorganic composite membranes offer excellent wettability by the electrolytes, good thermal stability and zero-dimensional shrinkage at high temperatures, they are usually not mechanically strong enough to withstand handling in cell winding and assembly.
In particular, separators used in wound electrochemical cells require a high mix penetration strength to avoid penetration of electrode materials through the separator. If particulate materials from the electrodes penetrate the separator, a short circuit will result.
In many cases, the inorganic composite membrane contains a very high content of inorganic filler materials. In some instances, the inorganic composite membrane so obtained exhibits poor mechanical strength.
One particular challenge has been thus to provide for multi-layer composite membranes with acceptable thickness to be suitably used as separators in electrochemical devices.
Multilayer composite membranes can be obtained using multiple coating steps. However, multiple steps disadvantageously increase processing costs.
There is thus still the need in the art for an alternative process for manufacturing solid composite separators and for solid composite separators having high porosity and thus high ionic conductivity to be suitably used as separators in electrochemical devices while maintaining outstanding thermo-mechanical properties during assembly and/or operation of the same.